Receiver connection in electroacoustic duplex system



DUPLEX SYSTEM Filed Jan. 20, 1945 O. Ts chuzz/zl Patented Feb. 15, 1949OFFICE 1 RECEIVER CONNECTION IN ELECTRO- ACOUSTIC DUPLEX SYSTEM fOttoTschumi, SoleurefiSwitzerland, assignor to the firm AutophonAktiengesellschaft, Soleure, Switzerland, a joint-stock company ofSwitzerland Application January 20, 1945, Serial No. 573,691 InSwitzerland November 3, 1943 Section 1,-Public Law .690, August 8, 1946Patent expires November 3, 1963 I 2 Claims. (Cl. 179-1) f The, presentinvention relates to duplex telephone. systems utilizing a transducercapable of serving 'both as a microphone and as a loudspeaker.

It 'is an object of the invention to provide in the circuit at thesubscribers stations, a headset receiver so constructed and arranged asto enable the conversation to be followed under certain conditionswithout Producing detrimental effects, for example, acoustic reactioneffects, in the circuit.

Theobjects and advantages of the invention will be more fully understoodfrom the following description of twoembodiments of the invention whichare shown by way of example in theaccompanying drawings.

In the drawings:

*Fig. 1 is a diagram of the subscribers end ofa telephone systemembodying my invention.

Fig. '2 is a similar diagram of another embodiment.

Referring first to the embodiment shown in Fig. 1, the transducer LMworks as a microphone, or as a loud-speaker according to whether it isconnected to the input or to the output of an amplifier V. Thechange-over from transmission to reception and vice versa is effected byswitches ul and U2, which may be manually operated or controlled by asuitable relay (not shown). With the circuitin condition fortransmission of a signal, one lead of the transducer LM, acting as amicrophone, is connected by switch ul with the input transformer E ofthe amplifier. V, the other lead going to ground. The output transformerA is connected through the switch 142 and through a switch AS with ahead set receiver H, which is located at the subscribers station.

In accordance with the present invention, the lead from the amplifier isconnected to the midpoint of the winding of the receiver H. One end ofthe said winding is connected through the distribution point a with onewinding "of a'transformer LU, the other winding of which is connectedwith the terminals al and bl of the line. The other end of the windingof the head set H is connected through a balance circuit N to ground.The balance N consists of a circuit, the impedance of which correspondwith that of the transformer LU loaded by the subscriber's line.

The winding of the head set receiver H is thus in efiect divided intotwo parts, hi and M, the lead from the amplifier V being connected tothe mid point between the two parts of the winding. The impedance of thepath from the mid point of said winding, through the winding part hi,the transformer LU, to ground, is the same as that of the path from themid-point of the winding through the winding part b2, and the balancecircuit N to ground. Hence, the amplified signal current from thetransducer LM acting as a microphone is distributed into two equalportions in the receiver H, half of it flowing through the coil hiandthe other half flowing through the coil h2. Owing to the fact thatthe two coil halves are transversed by equal currents flowing inopposite directions, the induction of the coil h! and the induction ofthe coil or neutralize one another. Because of this neutralizing effect,no signal is produced in the receiver H. Hence the signals transmittedby the transducer acting as a microphone are transmitted through theamplifier V, the coil hi and the transformer LU to the line terminals aland bl, but because of the neutralizing effect of the two coils h! andM, these signals are not per- "ceptible in the head set H. Interferenceor" reaction between the head set and the transducer is thus avoided.

When the circuit of Fig. 1 is in condition to receive a signal from theline, the switches ul and a2 are reversed. The signal is receivedthrough the terminals a! and bi which are connected with one winding ofthe transformer LU. One lead of the other winding of transformer LU .isconnected to ground. The other lead is connected both to the inputtransformer E of the amplifier V and to the head set H, the two being inparallel. The output of the amplifier V is connected by switch U2 withthe transducer LM, which now acts as a loud speaker. The other end ofthe winding of the head set H is connected to ground. Hence, a portionof the signal current from the transformer 'LU will flow through bothcoils h! and M of the head set H in series and through the balance N toground.

Another portion flows through the switch UI and the input transformer Eof the amplifier V to ground, the output transformer A of the amplifierbeing connected by the switch a2 with the transducer LM. A signalreceived from the line (al, bl) through the transformer LU will hence bereceived both by the transducer acting as a loud speaker and by the headset H. It is hence possible to receive a Weak signal on the head-set H.

The circuit shown in Fig. 2 is basically the same as that of Fig. 1 andthe corresponding parts are designated by the same reference characters. It difiers from the circuit of Fig. 1 in 2,461,945 I i v.: l

that the location of the distribution point a is difierent, beinglocated in the lead between the head set receiver H and the amplifier V.

The arrangement shown in Fig. 2 is especial- ,ly advantageous when thesubscribers station is working as a receiver and the sound energy outputof the loud-speaker LM is so high as to cause the head-set receiver H towork as a microphone. In these circumstances acoustic reaction mightoccur in the circuit of Fig. 1, since a part, of the current created bythe head-set H acting as a microphone could flow through the windingcoil hi, the distribution point a. and the changeover switch ui, to theamplifier V.

This is prevented in the arrangement of Fig. 2, by locating thedistribution point a in the lead between the amplifier V and the centralleadin point of the winding of the head-set H. With this arrangementthere exists an equalized bridgeconnection, the arms of which are formedby the coils hi and 712 of the head-set winding and by the balance N andthe subscribers winding of the transformer LU. Hence, any signal currentresulting from action of the head-set H as.

a microphone produces no potential difierence between the distributionpoint a and ground, and consequently no signal current from the headsetH will fiow through the input transformer E of the amplifier. Acousticreaction between the transducer LM and the head-set H is therebyprevented.

What I claim is:

1. In an electro-acoustic circuit, the combination With a transducer, anamplifier having an input and an output, a telephone receiver having awinding, a. transformer for coupling the transducer circuit to anoutside line and switching means connecting the transducer alternatelyto the input of said amplifier for transmission and to the output forreception of a signal, of a balance circuit, means including saidswitching means connecting the input of the amplifier with 4 saidtransformer when said switching means is in position for reception of asignal by the transducer, and means including said'switching meansconnecting the output of said amplifier with the midpoint of the windingof said receiver when said switching means is in position fortransmission of a signal by the transducer, one end of said windingbeing connected with one terminal of one of the windings of saidtransformer and the other end being connected through said balancecircuit with the other terminal of said transformer winding, theimpedance of said balance circuit being substantially identical with theeffective impedance of said transformer winding whereby output currentfrom the amplifier is divided substantially equally between the twohalves of the winding of the telephone receiver and flows in oppositedirections through said halves.

2. A circuit as claimed in claim 1, in which the means for connectingthe input of the amplifier with the transformer comprises a connectionbetween the input of the amplifier and the midpoint of said telephonereceiver winding.

OTTO TSCHUMI.

REFERENCES CITED The following references are of record in the file ofthis patent:

